Architecture

Comparison Process

This is the overview of the comparison process as a whole. Each of the six phases of the comparison process of EMF Compare are briefly defined on the Overview, and a much more in-depth explanation will be given below, in our explanations of the default behavior of EMF Compare.

Project Architecture

EMF Compare is built on top of the Eclipse platform. We depend on the Eclipse Modeling Framework (EMF), the Eclipse Compare framework and, finally, Eclipse Team, the framework upon which the repository providers (EGit, CVS, Subversive...) are built.

Whilst we are built atop bricks that are tightly coupled with the eclipse platform, it should be noted that the core of EMF Compare can be run in a standalone application with no runtime dependencies towards Eclipse; as can EMF itself.

The Comparison Model

EMF Compare uses a single model, which root is a Comparison object, to represent all of the information regarding the comparison: matched objects, matched resources, detected differences, links between these references, etc. The root Comparison is created at the beginning of the Match process, and will undergo a set of successive refinings during the remainder of the Comparison: Diff, Equivalence, Dependencies... will all add their own information to the Comparison.

So, how exactly is represented all of the information the Comparison model can hold, and how to make sense of it all?

Match

A Match element is how we represent that the n compared versions have elements that are basically the same. For example, if we are comparing two different versions v1 and v2 of a given model which look like:

Master

Borrowables

Comparing these two models, we'll have a Comparison model containing three matches:

library <-> library

Book <-> Novel

title <-> title

In other words, the comparison model contains an aggregate of the two or three compared models, in the form of Match elements linking the elements of all versions together. Differences will then be detected on these Match and added under them, thus allowing us to know both:

what the difference is (for example, "attribute name has been changed from Book to Novel"), and

what the original elements were.

Diff

Diff elements are created during the differencing process in order to represent the actual modifications that can be detected within the source model(s). The Diff concept itself is only there as the super-class of the three main kind of differences EMF Compare can detect in a model, namely ReferenceChange, AttributeChange and ResourceAttachmentChange. We'll go back to these three sub-classes in a short while.

Whatever their type, the differences share a number of common elements:

a parent match: differences are detected on a given Match. Having a difference basically means that one of the elements paired through this Match differs from its "reference" side (see source description below).

a source: differences are detected on one side of their match. The source really only holds meaning in three-way comparisons, where a difference can be detected in either right or left. All differences detected through two-way comparisons have their source in the left side. This is because we always compare according to a "reference" side. During two-way comparisons, the reference side is the right: differences will always be detected on the left side as compared with the right side. During three-way comparisons though, differences can be detected on either left or right side as compared with their common ancestor; but never as compared to themselves (in other words, this is roughly equivalent to two two-way comparisons, first the left as compared to the origin, then the right as compared to the origin).

a current state: all differences start off in their initial unresolved state. The user can then choose to:

merge the difference (towards either right or left, applying or reverting the difference in the process), in which case the difference becomes merged, or

discard it, thus marking the change as discarded. For example, if there is a conflicting edit of a textual attribute, the user can decide that neither right nor left are satisfying, and instead settle for a mix of the two.

a kind: this is used by the engine to describe the type of difference it detected. Differences can be of four general types:

Add: There are two distinct things that EMF Compare considers as an "addition". First, adding a new element within the values of a multi-valued feature is undeniably an addition. Second, any change in a containment reference, even if that reference is mono-valued, that represents a "new" element in the model is considered to be an addition. Note that this second case is an exception to the rule for change differences outlined below.

Delete: this is used as the counterpart of add differences, and it presents the same exception for mono-valued containment references.

Change: any modification to a mono-valued feature is considered as a change difference by the engine. Take note that containment references are an exception to this rule: no change will ever be detected on those.

Move: once again, two distinct things are represented as move differences in the comparison model. First, reordering the values of a multi-valued feature is considered as a series of MOVE: one difference for each moved value (EMF Compare computes the smallest number of differences needed between the two sides' values). Second, moving an object from one container to another (changing the containing feature of the EObject) will be detected as a move.

In order to ensure that the model stays coherent through individual merge operations, we've also decided to link differences together through a number of associations and references. For example, there are times when one difference cannot be merged without first merging another, or some differences which are exactly equivalent to one another. In no specific order:

dependency: EMF Compare uses two oppposite references in order to track dependencies between differences. Namely, requires and requiredBy represent the two ends of this association. If the user has added a package P1, then added a new Class C1 within this package, we will detect both differences. However the addition of C1 cannot be merged without first adding its container P1. In such a case, the addition of C1requires the addition of P1, and the later is requiredBy the former.

refinement: this link is mainly used by extensions of EMF Compare in order to create high-level differences to hide the complexity of the comparison model. For example, this is used by the UML extension of EMF Compare to tell that the three differences "adding an association A1", "adding a property P1 in association A1" and "adding a property P2 in association A1" is actually one single high-level difference, "adding an association A1". This high-level difference is refinedBy the others, which all refines it.

equivalence: this association is used by the comparison engine in order to link together differences which are equivalent in terms of merging. For example, Ecore has a concept of eOpposite references. Updating one of the two sides of an eOpposite will automatically update the other. In such an event, EMF Compare will detect both sides as an individual difference. However, merging one of the two will trigger the update of the other side of the eOpposite as well. In such cases, the two differences are set to be equivalent to one another. Merging one difference part of an equivalence relationship will automatically mark all of the others as merged (see state above).

conflict: during three-way comparisons, we compare two versions of a given model with their common ancestor. We can thus detect changes that were made in either left or right side (see the description of source above). However, there are cases when changes in the left conflict with changes in the right. For example, a class named "Book" in the origin model can have been renamed to "Novel" in the left model whereas it has been renamed to "Essay" in the right model. In such a case, the two differences will be marked as being in conflict with one another.

PENDING all other concepts of the core metamodel.

Core Concepts

Proxy Resolution

PENDING why does EMF Compare avoid proxy resolving, how?

Equality Helper

PENDING what's this?

Comparison Scope

As seen above, EMF Compare consider proxies as real citizens of the EMF realm. This mainly shows in the matching mechanism. EMF Compare uses a scoping mechanism to determine which elements should be matched together, and which others should be ignored. Any element that is outside of the comparison scope will be ignored by the comparison engine and left alone (if it is a proxy, it won't even be loaded). This also means that we won't really have a way to compare these proxy (or otherwise out-of-scope values) when the Diff process encounters them.

For example, an element that is outside of the comparison scope, but referenced by another element which is in the scope will need specific comparison means: we've ignored it during the matching phase, so we don't know which 'out-of-scope' element corresponds to which 'other out-of-scope' element. Consider the following: in the first model, a package P1 contains another package P2. In the right, a package P1' contains a package P2' . We've told EMF Compare that P2 and P2' are out of the comparison scope. Now how do we determine that the reference from P1 to P2 has changed (or, in this example, that it did not change)?

This is a special case that is handled by the IEqualityHelper. Specifically, when such cases are encountered, EMF Compare falls back to using the URI of the two objects to check for equality. This behavior can be changed by customizing the IEqualityHelper (see above).

By default, the only thing that EMF Compare considers "out of scope" are Ecore's "EGenericType" elements. These are usually meaningless as far as comparison is concerned (as they are located in derived references and will be merged along with their "true" difference anyway). Please take note that, when used from the user interface, EMF Compare will narrow down the scope even further through the resolution of the logical model and determining which resources are actually candidates for differences.

The comparison scope provides EMF Compare with information on the content of ResourceSets, Resources or EObjects, according to the entry point of the comparison. Take note that the scope is only used during the matching phase. The differencing phase only uses the result of the matching phase to proceed.

Longest Common Subsequence

PENDING description of the algorithm, why do we use it, references

Default behavior and extensibility

All main components of EMF Compare have been designed for extensibility. Some are only extensible when comparing models through your own actions, some can be customized globally for a given kind of model or metamodel... We'll outline the customization options of all 6 comparison phases in this section. (Any dead link? Report them on the forum!)

Model Resolving

PENDING description of the phase, extensibility (use of the modelProviders extension point, custom ext point of compare)

Match

Before we can compute differences between two versions of a same Object, we must determine which are actually the "same" Object. For example, let's consider that my first model contains a Package P1 which itself contains a class C1; and that my second model contains a package P1 which contains a class C1. It may seem obvious for a human reader that "P1" and "C1" are the same object in both models. However, since their features might have changed in-between the two versions (for example, the "C1" might now be abstract, or it could have been converted to an Interface), this "equality" is not that obvious for a computer.

The goal of the "Match" phase is to discover which of the objects from model 2 match with which objects of model 1. In other words, this is when we'll say that two objects are one and the same, and that any difference between the two sides of this couple is actually a difference that should be reported as such to the user.

By default, EMF Compare browses through elements that are within the scope, and matches them through their identifier if they have one, r through a distance mechanism for all elements that have none. If the scope contains resources, EMF Compare will first match those two-by-two before browsing through all of their contained objects.

EMF Compare "finds" the identifier of given object through a basic function that can be found in IdentifierEObjectMatcher.DefaultIDFunction. In short, if the object is a proxy, its identifier is its URI fragment. Otherwise its functional ID (in ecore, an attribute that serves as an identifier) takes precedence over its XMI ID (the identifier it was given in the XMI file). If the object is not a proxy and has neither functional nor XMI identifier, then the default behavior will simply pass that object over to the proximity algorithms so that it can be matched through its distance with other objects.

Changing how resources are matched

By default, the logic EMF Compare uses to match resources together is very simple: if two resources have the same name (strict equality on the name, without considering folders), they match. When this is not sufficient, EMF Compare will look at the XMI ID of the resources' root(s). If the two resources share at least one root with an equal XMI ID, they match.

Defining custom identifiers

In some cases, there might be ways to identify your objects via the use of "identifiers" that cannot be identified as such by the default mechanism. For example, you might want each of your objects to be matched through their name alone, or through the composition of their name and their type... This can be achieved through code by simply redefining the function EMF Compare uses to find the ID of an object. The following code will tell EMF Compare that the identifier of all "MyEObject" elements is their name, and that any other element should go through the default behavior.

Refine the default Match result

If you are happy with most of what the default behavior does, but would like to refine some of it, you can do so by post-processing the result of the match phase. The original models are only used when matching, and will never be queried again afterwards. All remaining phases are incremental refinings of the "Comparison" model that's been created by the matching phase.

As such, you can impact all of the differencing process through this. Within this post-processing implementation, you can:

Remove Match elements: no difference will be detected on those: neither additions, nor deletions, nor conflicts... They'll simply be entirely ignored by the remaining process. Do note that elements for which we have no match will be considered "distinct" by the innards of EMF Compare: if a couple "B<->B'" references a couple "C<->C'" through one of their references, but you have removed the Match "C<->C'", we will considered that this reference has been "changed" from C to C' and this difference within the references of B will be shown as such.

Add new Match element

the new couples of elements will be considered by the remaining comparison process and difference may be detected on them.

Change existing Match elements

unmatched elements have two or three associated Match objects. For example if you are comparing three version of a model which all contain a different version of a given package, and all three version change the name of this package: version 1 has package "P1", version 2 has package "P2" and version three has package "P3". This package is actually the same, but EMF Compare did not manage to match it. We will thus have three Match objects: one that references "P1" as left, one that references "P2" as right and one that references "P3" as origin.

You may remove two of those three elements and change the third one so that it references P1 as left, P2 as right and P3 as origin. In such a case, those three will be considered to Match for the remainder of the comparison process. Make sure that there are not two different Match referencing the same object though, as this would yield unspecified results.

Defining a custom post-processor requires you to implement IPostProcessor and registering this sub-class against EMF Compare. The latter can be done via either an extension point, in which case it will be considered for all comparisons on models that match its enablement, or programmatically if you only want it active for your own actions:

Through code

The following registers a post-processor for all UML models. This post-processor will not be triggered if there are no UML models (matching the given namespace URI) within the compared scope.

This accomplishes the exact same task, but it registers the post-processor globally. Any comparison through EMF Compare on a scope that contains models matching the given namespace URI will trigger that post-processor.

Diff

Now that the Matching phase has completed and that we know how our objects are coupled together, EMF Compare no longer requires the two (or three) input models. It will no longer iterate over them or the comparison's input scope. From this point onward, only the result of our comparison, the Comparison object, will be refined through the successive remaining phases, starting by the Diff.

The goal of this phase is to iterate over all of our Match elements, be they unmatched (only one side has this object), couples (two of the three sides contain this object) or trios (all three sides have this object) and compute any difference that may appear between the sides. For example, an object that is only on one side of the comparison is an object that has been added, or deleted. But a couple might also represent a deletion: during three way comparisons, if we have an object in the common ancestor (origin) and in the left side, but not in the right side, then it has been deleted from the right version. However, this latter example might also be a conflict: we have determined that the object has been removed from the right side... but there might also be differences between the original version and the "left" version.

The differencing phase does not care about conflicts though: all it does is refine the comparison to tell that this particular Match has n diffs: one DELETE difference on the right side, and n differences on the left. Detecting conflicts between these differences will come at a later time, during the conflict resolution phase.

Customizations of this phase usually aim at ignoring specific differences.

Changing the FeatureFilter

One of the differencing engine's responsibilities is to iterate over all features of a given object in order to check for potential differences on its value(s). However, there are some features that we decide do ignore by default: derived features, transient features... or some features on which we would like to check for ordering changes even though they are marked as non-ordered.

The logic to determine whether a feature should be checked for differences has been extracted into its own class, and is quite easy to alter. For example, if you would like to ignore the name feature of your elements or never detect any ordering change:

You could also change the diff processor to achieve a similar goal. The difference between the two approaches is that changing the FeatureFilter will ignore the structural feature altogether, whereas replacing the diff processor would let EMF Compare check the feature and detect that diff, but ignore the notification that there is a change.

Changing the Diff Processor

The diff engine browses over all of the objects that have been matched, and checks all of their features in order to check for changes between the two (or three) versions' feature values. When it detects a change, it delegates all of the corresponding information to its associated Diff Processor, which is in charge of actually creating the Diff object and appending it to the resulting Comparison.

Replacing the Diff Processor gives you a simple entry point to ignore some of the differences the default engine detects, or to slightly alter the Diff information. You might want to ignore the differences detected on some references for example. Or you might want to react to the detected diff without actually creating the Comparison model... The implementation is up to you. You can either reimplement the whole contract or extend the default implementation, DiffBuilder

Here is a simple example that provides EMF Compare with a diff processor that will ignore all differences detected on the "name" attribute of our objects, yet keep the default behavior for all other differences.

Refine the default Diff result

The last possibility offered by EMF Compare to alter the result of the differencing phase is to post-process it. The remaining comparison phases -equivalence detection, detection of dependencies between diffs and conflict detection- all use the result of the Diff engine and refine it even further. As such, all of these phases can be impacted through the refining of the Diff result.

Example uses of the post-processing would include:

Remove Diff elements

If you'd rather code your logic to ignore differences here as a post-process instead of changing the FeatureFilter or IDiffProcessor. Though that is not the best way to ignore differences, it can still be done here.

Add new Diff elements

If you want to create new differences without implementing a whole differencing engine, new differences can still be created here as a post-process. You will need to implement the iteration over model elements and specific checks yourself though. This workaround can also be used to create new differences that the default differencing engine does not know.

Alter the detected differences

If some of the differences have been detected in a way you do not like, and you did not use a custom IDiffProcessor to change the Diff information, you can do so here.

The post-processor for the diff engine is implemented exactly in the same way as for the match engine post-processors (the interface and extension point are the same). Please refer to Refining the Match result.

User Interface

Using The Compare APIs

The main entry point of EMF Compare is the org.eclipse.emf.compare.EMFCompare class. It is what should be used in order to configure and launch a comparison. That is not all though. Once you have compared two models, you want to query the differences, maybe merge some of them, display the comparison result in the comparison editor... The following section will list the main entry points for each of these actions, along with a brief description of what can be done and what should be avoided.

Most of these examples are set-up as "standalone" examples, and will include extra instructions for IDE use: pay attention to the environment in which you are using EMF Compare. Are you using it from an Eclipse plugin, in which case you'd like all of the extra features provided through extension points and contribution to be available to you? Or are you using it from a standalone environment, in which case you'd need to reduce the dependencies to the bare minimum and avoid OSGi-related code and extensions?

Compare two models

Loading your models

Whether you wish to compare two or three models, the first thing you need is to load them. We won't detail in-depth how to do that as this is standard EMF practice, you might want to look at EMF tutorial for detailled instructions on this point. Here, we'll use a simple method that loads an xmi file at a given URL into a resourceSet, expecting the given URL to be an absolute file URL:

Creating the comparison scope

EMF Compare uses a scoping mechanism to determine which elements should be compared, and which others should be ignored. Any element that is outside of the comparison scope will be ignored by the comparison engine and left alone (if it is a proxy, it won't even be loaded). As such, extra care should be taken to determine the proper scope of the comparison or customize the IEqualityHelper to handle the specific elements you remove from the scope. Refer to the appropriate section above for more on the scoping mechanism.

By default, the only thing that EMF Compare considers "out of scope" are Ecore's "EGenericType" elements. These are usually meaningless as far as comparison is concerned (as they are located in derived references and will be merged along with their "true" difference anyway). Other than that, Please note that EMF Compare will leave unresolved proxies alone: more on this can be found in the related section.

Configuring the comparison

EMF Compare can be customized in a number of ways, the most important of which were described above. Most of them re-use the same entry point, the org.eclipse.emf.compare.EMFCompare class. We won't customize much here, please see the afore-mentionned section for extensibility means.

All we will tell EMF Compare is not to use identifiers, and rely on its proximity algorithms instead (after all, we're comparing plain XMI files):

Comparing from an Eclipse plugin

The above example is for standalone usage, and as such will require extra work if you wish to compare UML models, benefit from EMF Compare extensions, provide your own mergers... The following represents the same example, but uses IDE-specific utilities (can you spot the two differences?):

Query the differences

Once you have the result of a comparison (in the form of a Comparison object), what you are interested in are most likely the differences between your models. We will detail the merging process later on it its own section, but before anything we need to retrieve the list of differences of interest. Within the Comparison model, differences are spread under the elements on which they've been detected, more precisely, under the Match of the element on which they were detected.

Let's use a complex example as reference. Consider the three following models:

Origin

Left

Right

All differences

What we need is usually to retrieve the list of all differences, wherever they were detected, or whatever their source (the left model, or the right model). Instead of iterating all over the Comparison model to collect them, you can use:

List<Diff> differences = comparison.getDifferences();

Differences related to element X

Sometimes, we need to retrieve all of the differences that were detected on (or that are related to) a given model element. For example, with the above example, we might want to retrieve the list of all differences that relate to Borrowable. Well, there are a number of them, which can all be collected through:

// borrowable is a reference on the like-named EObject
List<Diff> differencesOnBorrowable = comparison.getDifferences(borrowable);

This will return a list containing a number of differences:

Borrowable has been added in the right model

copies has been added to reference ownedProperties of Borrowable

Borrowable has been added to the generalization reference of Book

Borrowable has been added as the borrowed target of an association with Person

In other words, this method will return differences under the target (here, copies has been added), as well as differences which changed value is the target.

Filtering differences

EMF Compare depends on guava for many of its internals. A number of "common" difference filtering predicates have been extracted to the org.eclipse.emf.compare.utils.EMFComparePredicates utility class. Using this class, it is trivial to filter the list of differences so as to keep only those we are interested in. For example, what if we wish to retrieve the list of all non-conflictual differences that originate from the left side? (This is the case when you use the "copy all non-conflicting from left to right" action from the comparison editor for example.)

We strongly encourage you to look around more in these classes: Predicates provides a number of basic, general-purpose predicates while EMFComparePredicates provides EMF Compare specific predicates used throughout both core and user-interface of EMF Compare.